A coil of teeth caps the lower jaw of a sculpture of a 13-foot (4-meter) whorl-tooth shark, or Helicoprion, a fish genus that lived about 250 million years ago.

Artist Gary Staab depicts the animal's jaw as something of a spiral conveyor belt, in which new teeth would advance to replace old ones (concealed here by skin) . But the true arrangement and purpose of the teeth remains a mystery. Some scientists suggest that it may have operated like a spiked whip, possibly curled underneath the lower jaw like a weaponized elephant trunk.

The shark adds bite to "Bizarre Beasts, Past and Present," a new exhibition of Staab's sculptures at the National Geographic Museum in Washington, D.C. (through February 2, 2007). The animals depicted are, or were, all realtestaments to the twists, turns, and blind alleys of evolution.

(Both National Geographic News and the National Geographic Museum are parts of the National Geographic Society.)